The present invention relates to a vehicle control system for actively controlling either or both of the front and rear wheel steer angles.
Recent examples of the steering control systems are shown in Japanese Pat. application No. 62-121015, Japanese Pat. application No 62-120261.
These control systems control the rear wheel steer angle in accordance with a steering angle, a steering angular speed and a steering angular acceleration.
These control systems calculate the rear wheel steer angle by using a mathematical linear combination including a term proportional to the steering angular speed and a term proportional to the steering angular acceleration. Because of this feature, these systems tend to cause unstable behavior of the controlled vehicle especially during steady state operations such as straight ahead operation and steady state turning operation. In steady state operation, the steering wheel is held fixed. However, these control systems are highly sensitive even to fluctuation of the steering wheel due to unconscious operation of the driver. In FIG. 9, the dashed lines show fluctuations in the rear wheel steer angle and vehicle lateral displacement produced in response to fluctuation in the steering wheel angle during straight ahead driving.